Series-parallel reactions examples

The following example illustrates a combination of semibatch and semicontinuous operation for an irreversible reaction, with one reactant added intermittently and the other flowing (bubbling) continuously, that is, a combination of Figures 12.3(a) and 12.4(a). Chen (1983, pp. 168-211, 456-460) gives several examples of other situations, including reversible, series-reversible, and series-parallel reactions, and nonisothermal and autothermal operation. 

For the general series-parallel reaction we introduce two additional considerations. First of all for parallel steps if one requirement is for a high temperature and another is for a low temperature, then a particular intermediate temperature is best in that it gives the most favorable product distribution. As an example, consider the reactions... 

Figure 9.22 Example of series parallel reaction in a batch reactor. Temperature on left scale, selectivity on right scale.

Levenspiel (8) discussed these examples and has proposed the following very useful rule of thumb Series-parallel reactions can be analyzed in terms of their constituent series reactions and parallel reactions in that optimum contacting for favorable product distribution is the same as for the constituent reactions. 

The plug-flow reactor is generally accepted as the most favorable for intermediate selectivity in series-parallel reactions. The results of this example clearly show that the membrane reactor can significantly outperform the PFR. 

Series-parallel reactions are reactions that are combinations of both series and parallel reactions. For example... 

We will now give three examplc.s of multiple reactions with heat effects Example 12-5 discusses parallel reactions. Example 12-6 discusses series reactions, and Example 12-7 discusses complex reactions. 

Chain-growth polymerization is perhaps the most important and complex example of a mixed series/parallel reaction. For example, polystyrene is formed by adding one styrene molecule at a time to the end of a growing polymer chain. 

That the reaction with a lower rate constant is taking place preferentially and that the rate increases during the reaction are phenomena that can also occur with parallel reactions. As an example, Wauquier and Jungers (48), when studying competitive hydrogenation of a series of couples of aromatic hydrocarbons on Raney-nickel, have observed these phenomena for the couple tetraline-p-xylene (Table I). The experimental result was... 

This reaction set may be regarded as parallel reactions with respect to consumption of species B and as a series reaction with respect to species A, V, and W. Common examples include the nitration and halogenation of benzene and other organic compounds to form polysubstituted compounds. To characterize the qualitative behavior of such systems, it is useful to consider reactions 9.3.3 and 9.3.4 as mechanistic equations and to analyze the effects of different contacting patterns on the yield of species V. We shall follow the treatment of Levenspiel (7). 

Table XV lists the isokinetic temperatures of several reactions representing a wide variety of mechanisms, these examples having been chosen because the isokinetic temperature happened to fall in the popular experimental range between 0 and 100°. There are many other polar reactions that have isokinetic temperatures well outside of the accessible temperature range there are many whose variations in activation energy and entropy are not parallel and these, of course, do not have an isokinetic temperature even approximately. When one of a series of reactions deviates markedly from a parallel trend in activation energy and entropy established by the others, it is probable that it differs in mechanism from the others. This is a better indication of a change in mechanism than either marked differences in rate or in activation energy.

A reaction network, as a model of a reacting system, mas7 consist of steps involving same ar all of opposing reactions, which may or may not be considered to be at equilibrium, parallel reactions, and series reactions. Some examples ate dted in Section 5.1. 

Figure 18.4 Vessel configuration for parallel reactions in Example 18-6 (a) single PER (b) three PFRs of same total volume in series with FBo split evenly among them...

The fact that reaction (12) is much slower than reaction (8), implies that Fe is faster depleted from the solution. As a result, Fenton process is halted because the redox chain cannot be supported itself. In addition, it is accepted that (Pignatello 1992 Boye et al. 2003) the hydroperoxyl radical (HO2 ) has a much lower oxidant power than OH. In the presence of organics, Fenton chemistry is even more complex because hydroxyl radical, both iron cations and the oxidation products enter into a series of consecutive and parallel reactions. An example of the complexity of these reactions is discussed elsewhere (Gozzo 2001) but a brief description is given here. The initial step for an organic substrate (R-H) oxidation starts with the interaction of itself with OH, according to (Walling and Kato 1971) ... 

We could write species mass-balance equations (S = 6 in this example) on any such reaction sequence and solve these (/ = 4 are inseparable) to find Cj x), and in most practical examples we must do this. However, there are two simple reaction networks that provide insight into these more complex networks, and we wiU next consider them, namely, series and parallel reaction networks (Figure 4-3). 

Parallel Pathways for Amino Acid and Fatty Acid Degradation The carbon skeleton of leucine is degraded by a series of reactions closely analogous to those of the citric acid cycle and j8 oxidation. For each reaction, (a) through (f), indicate its type, provide an analogous example from the citric acid cycle or /3-oxidation pathway (where possible), and note any necessary cofactors. 

As an example of a system with a series of reactions, we may look at methane oxidation under conditions of excess oxygen. Following the carbon atom, this process would typically involve the steps CH4 — CH3 — CH2O — HCO — CO —> CO2. We note that each of these steps may involve a number of parallel elementary reactions, but we assume that they do not affect the oxidation pathway. 

The series-parallel type of reaction outlined in Section 1.10.1 is quite common among industrial processes. For example, ethylene oxide reacts with water to give monoethylene glycol, which may then react with more ethylene oxide to give diethylene glycol. 

In a recent example Kappe and co-workers112 scaled up a series of reactions from a single mode to a multimode parallel batch reactor. Typically reactions were scaled from 1 to 100 mmol. The transformations included... 

Finally, the eighth reaction mechanism in Table 2.1 includes both series and parallel reactions to the same product P. This scheme is more complete and somewhat more realistic, but it is not so much different from the series scheme, because the side parallel reaction to P only produces small changes in the shape of the concentration profiles. As an example, the initial zero derivative for Cp can be canceled. 

Multiple reactions Series or parallel reactions that take place simultaneously in a reactor. For example, A -(- B C and A + D - E are parallel reactions, and A + B C + D E + F arc series reactions. 

When a reaction rate is measured in a chemical reactor, the reaction is generally a composite reaction comprised of a sequence of elementary reactions. An elementary reaction is a reaction that occurs at the molecular level exactly as written (Laidler, 1987). The mechanism of the reaction is the sequence of elementary reactions that comprise the overall or composite reaction. For example, mineral dissolution reactions generally include transport of reactant to the surface, adsorption of reactant, surface dilfusion of the adsorbate, reaction of the surface complex and release into solution, and transport of product species away from the surface. These reactions occur as sequential steps. Reaction of surface complexes and release to solution may happen simultaneously at many sites on a surface, and each site can react at a different rate depending upon its free energy (e.g., Schott et al., 1989). Simultaneous reactions occurring at different rates are known as parallel reactions. In a series of sequential reactions, the ratedetermining step is the step which occurs most slowly at the onset of the reaction, whereas for parallel steps, the rate-determining step is the fastest reaction. 

The reaction of diazoalkanes with acetylenes can give rise to cyclopropenes by two main routes. Some reactions involve an initial loss of nitrogen to generate a carbene which then adds to the acetylene (see Section 1.2.1.), but this section is concerned only with those reactions where the first step is a cycloaddition leading to formation of a 3//-pyrazole. Unlike the parallel series of reactions in the cyclopropane series, where the C-C double bond of the alkene requires activation by a suitable substituent or by strain. Under pressure even acetylene itself will react with diazoalkanes. For example, diphenyldiazomethane underwent addition in good yield and deazetization gave 3,3-diphenylcyclopropene (1). ... 

Solution This is an example of series-parallel reversible reactions. The reactor design formulation of diese chemical reactions was discussed in Example 4.1. Here, we complete die design for an isothermal plug-flow reactor and obtain the reaction and species curves. Recall that we select Reactions 1, 3, and 5 as a set of independent reactions. Hence, the indices of the independent reactions are m = 1, 3, and 5, the indices of the dependent reactions are = 2, 4, and 6, and we express the design equations in terms of Zi, Z3, and Z5. The stoichiometric coefficients of the three independent reactions are... 

Most poisons are type (1), i.e., independent compounds present tn the feed, perhaps in minute quantities, that deactivate the site with a mechanism different from the main reaction. Examples are also found of types (2) and (3), where either parallel or series reactions generate side products that poison the sites. These mechanisms may also be classified as examples of kinetic inhibition but are considered poisoning if adsorption on the site is irreversible. In situations where multiple sites are involved (for example, dual-functional catalytic reforming), poisoning patterns become more complex. 

Examples of simple reactions (such as considered in Case Study 11.7) are not hard to find in industry, but those of complex reactions are more widespread. One can envisage systems with increasing complexities such as the manufacture of nitric acid. The complexities include the following (1) multiple (parallel) reactions by the same reactant, (2) reactions in series, (3) multiple reactions among multiple reactants, (4) reversible reactions, (5) reactions in multiple phases, and... 

In foods, we often have what may be called reaction cascades, i.e., a whole series of reactions, partly consecutive, partly parallel, with bifurcations and with more than one reaction pathway leading to the same product. Examples are nonenzymatic browning or Maillard reactions, as well as several changes occurring during heat treatment. Chain reactions may be involved as well, as in the formation of hydroperoxides during the autoxidation of fats ... 

It can be proposed that degradation of a-, p-, and y-CDs follows a parallel series of reactions as shown with a-CD as an example in Figure 16.6. 

In this chapter, we discuss reactor selection and general mole balances for multiple reactions. First, we describe the four baste types of multiple reactions series, parallel, independent, and complex. Next, we define the selectivity parameter and discuss how it can be used to minimize unwanted side reactions by proper choice of operating conditions and reactor selection. We then develop the algorithm that can be used to solve reaction engineering problems when multiple reactions are involved. Finally, a number of examples are given that show how the algorithm is applied to a number of real reactions. 

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